Interesting question. I'm not sure - it doesn't seem intuitively like Twitter / Dorsey (as far as i can tell Vine is Dorsey's pet project) would stoop to that kind of stuff.
Why is a publicity stunt something low or to be frowned upon?
Well in this case, it exposes many unwilling/unexpecting people to pornography. That probably should be frowned upon.
Simply living on planet Earth in 2013 exposes many to pornography. Like anything we do in life, we have to take responsibility for ourselves. If that means being a jerk to our kids and making sure they don't use vine, then that's what that means. Same thing goes for taking the risk of looking at a video whose contents we aren't totally sure of.
It's just tacky. If I know anything about Dorsey it's that he's allergic to tacky